What are the differences between lamps in Cycles and Blender Internal?
Cycles uses a physically correct light falloff by default, Blender Internal uses a smoothed falloff with a Distance parameter. Cycles can give a similar falloff by using the Light Falloff node with Smooth parameter.

Cycles lamp strength for point, spot and area lamps is specified in Watts, while Blender Internal has no clear physical unit that I know of. For Cycles this means you typically need higher values, as you couldn't use a 1W lamp to light a room, you need something stronger like a 100W lamp. For Blender Internal the default Distance of 30 means you can use lower strength values.

Sun lamps in Cycles are specified in $\frac{\text{Watts}}{\text{m}^2}$, which requires much smaller values like 1 $\frac{\text{Watt}}{\text{m}^2}$. This can be confusing, but specifying strength in Watts wouldn't have been convenient, the real sun for example has strength 384600000000000000000000000W. Emission shaders on meshes are also in $\frac{\text{Watts}}{\text{m}^2}$.